For what may be the first time, outside groups — nominally independent spenders that are free from contribution limits and can sometimes conceal their donors from the public — are outspending the parties and candidates in ten key U.S. Senate races, according to this new analysis based on FEC data. While closely tied to the parties and candidates despite weak rules against coordination, these groups benefit from being able to raise most of their money from a handful of mega-donors. The analysis also finds outside spending reaching record highs, a disproportionate amount of it benefiting Republican candidates, and an increasing concentration of dark money among a few big outside groups.